1. Field of the Invention
The present invention relates to an ink jet printing system which forms an image by arranging dots on a print medium by using a print head that ejects ink in the form of droplets according to image data. More particularly, the present invention relates to a dot control method for preventing possible image impairments on the print medium caused by ejected ink droplets separating into main droplets and satellites.
2. Description of the Related Art
As information processing devices, such as copying machines, word processors and computers, and communication devices are coming into wider use, an ink jet printing apparatus has come to be known as one of output devices to print an image (information) generated by these devices. The ink jet printing apparatus forms an image by applying ink to a print medium. More specifically, the ink jet printing apparatus uses a print head having a plurality of integrated printing elements (also referred to as nozzles), each comprised of an ink ejection opening and an ink path to supply ink to the opening, and ejects ink from the printing elements according to a print signal. Further, a growing number of ink jet printing apparatus with a plurality of such print heads that can meet the requirements of color printing are coming into the market.
The ink jet printing system ejects an ink or recording liquid in the form of flying droplets onto a print medium, such as paper, to form dots on it. This system, since it is of a non-contact type, has an advantage of low noise. By increasing a density of ink ejection nozzles, a resolution of an image can be enhanced and a high speed printing realized. This printing system can also produce a high quality image on such print mediums as plain paper at relatively low cost without requiring any special processing such as development and fixing. An on-demand type ink jet printing apparatus in particular is considered promising because it can easily be upgraded to have a color printing capability and reduced in size and complexity.
In such an ink jet printing apparatus, there are growing demands in recent years for faster printing speed and higher image quality. To meet these demands, the nozzle integration technology has achieved a rapid advance and many long print heads with high densities of nozzles are available on the market. As the density of nozzles increases, a technology to reduce an amount of ink ejected from individual nozzles is also being developed. Further, a printing apparatus is also available which provides an improved greyscale of image by adopting a technique for ejecting multiple sizes of ink droplets from each nozzle or a construction in which a plurality of nozzle columns are installed for each size of ink droplets to be ejected. To realize a faster printing speed, a technique has been developed that increases a frequency of ink ejection from the nozzles and moves the print head mounting carriage at a correspondingly faster speed.
When the ejection state of individual nozzles in the ink jet print head becomes unstable, it is generally known that an ink droplet ejected in one ejection operation separates into a main droplet and a smaller sub droplet. Since the main and sub droplets have different flying speeds, these two droplets that are ejected as the carriage moves land on different positions on a print medium. In the following description, dots formed by the main droplets are called main dots and those formed by sub droplets satellites. If satellites show too distinctively, dots are recognized at positions unrelated to image data, which may lead to an image problem. There are cases, however, where such satellites do not pose a problem if they are sufficiently small compared with the main dots or land very close to the main dots.
To cope with the problem of satellites, various methods have been proposed which include, for example, one that limits the nozzles used during reciprocal printing scans and one that uses different nozzles than those used for an outline portion of characters and figures where image impairments caused by satellites easily show. These techniques are disclosed, for example, in Japanese Patent Application Laid-open Nos. 06-135126, 2001-129981, 2002-086764, 2002-144608 and 07-304216.
It is noted, however, that an effort in recent years to reduce the size of ink droplets, though it has an effect of reducing the size of main droplets and minimizing the graininess of a printed image, can undesirably enhance the presence of the satellites. Further, increasing the traveling speed of the carriage for faster printing can pull farther apart the landing positions of the main and sub droplets that fly at different speeds, undesirably making the satellites more noticeable. The presence of satellites at positions unrelated to the image data can change the grayscale of an image, making the grayscale representation unstable. That is, as the image quality required of the ink jet printing apparatus goes higher and higher, the presence of the satellites and their adverse effects on the image are becoming an increasingly serious issue.
Especially, in a printing apparatus having a plurality of printing modes, a traveling speed of the carriage, a distance between an ink ejection face of the print head and a print medium (referred to simply as “distance to the paper”), or an ink ejection volume may vary from one mode to another. In this case, the distance between the main dot and the satellite is also unstable, which may cause image impairments to show depending on the print mode.